The invention relates to OFDM data transmission systems.
OFDM is a spread spectrum technology wherein the available transmission channel bandwidth is subdivided into a number of discrete channels or carriers that are overlapping and orthogonal to each other. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of carrier frequencies. The data transmitted over these OFDM symbol carriers may be encoded and modulated in amplitude and/or phase, using conventional schemes such as Binary Phase Shift Key (BPSK) or Quadrature Phase Shift Key (QPSK).
A well known problem in the art of OFDM data transmission systems is that of impulse noise, which can produce bursts of error on transmission channels, and delay spread, which often causes frequency selective fading. To address these problems, prior systems have utilized forward error correction (FEC) coding in conjunction with interleaving techniques. FEC coding adds parity data that enables one or more errors in a code word to be detected and corrected. Interleaving reorders the code word bits in a block of code word data prior to transmission to achieve time and frequency diversity.
Although the prior interleaving techniques can minimize some of the effects of impulse noise and delay spread on OFDM data transmission, they cannot mitigate the impact of a combination of impulse noise and frequency nulls, which may result in lengthy noise events.
The present invention features a channel estimation scheme for identifying usable carriers for a particular modulation type, and selecting, if possible, a modulation type for modulation of OFDM symbols in a standard transmission mode based on channel conditions associated with a data transmission received by a receiving network node over a data channel from a transmitting network node. The identified carriers and modulation type are made available to the transmitting network node for use in a next data transmission to the receiving network node over the data channel.
In one aspect of the invention, channel estimation is performed for an OFDM symbol block received by a receiving network node over a data channel from a transmitting network node by generating for available modulation types information indicative of noise events that occurred in the OFDM symbol block and determining from the generated information which of the available modulation types (including associated FEC coding rates) is to be used by the transmitting network node for a next data transmission over the data channel to the receiving network node.
Embodiments of the invention may include one or more of the following features.
The information may be generated across symbols and carriers in the OFDM symbol block.
The generation of information may include computing average symbol phase noise values for each of the symbols and average carrier phase noise values for each of the carriers.
The generation of information may further include determining one or more symbol counts, each corresponding to a different impulse noise symbol threshold and providing a number of average symbol phase noise values exceeding such different impulse noise symbol threshold.
The generation of information may further include determining carrier counts for the available modulation types from the computed average carrier phase noise values, each carrier count indicative of a number of the computed average carrier phase noise values exceeding a carrier threshold corresponding to a different one of the available modulation types. Each carrier count may be further indicative of a number of carrier values exceeding a jammer detection threshold.
The generation of information may further include generating for each of the available modulation types a corresponding channel map, the channel map identifying as xe2x80x9cgoodxe2x80x9d those carriers that contributed to the carrier counts for a one of the available modulation types to which the channel map corresponds.
The generation of information may further include generating average threshold margins for each of the carrier counts to indicate the average amount by which the corresponding carrier threshold was exceeded.
The carrier and one or more symbols counts may be used to determine if one of more of the available modulation types is acceptable for use with a standard transmission mode over the channel. The carrier and one or more symbol counts may be tested against criteria specified for each of the available modulation types.
Data rates of the one or more of the available modulation types determined to be acceptable for use with the standard transmission mode over the channel are computed. The fastest of the one or more available modulation types determined to be acceptable for use with the standard transmission over the channel may be selected.
The channel map corresponding to the selected modulation type may be provided to the transmitting network node.
An alternative, second transmission mode may be selected if it is determined that any of the modulation types for the standard transmission mode may not be selected. The second transmission mode may have an associated modulation type and be capable of a more robust transmission at a lower data rate than the modulation types used in the standard transmission mode.
The associated modulation type may be the slowest of the available modulation types.
The selected modulation type may be the same for all of the carriers.
The mechanism of the invention offers several advantages. The channel estimation produces a channel map that takes into account most recent characteristics and conditions of the channel as exhibited during a data transmission from a transmitting network node to a receiving network node. It identifies a modulation type and set of carriers to be used in a next data transmission from the transmitting network node to the receiving network node on that channel. The channel estimation mechanism attempts to select the highest data rate available given the channel conditions. If the requirements of a modulation type in a standard transmission mode cannot be satisfied to ensure reliable data transmission, the mechanism selects a lower data rate, but more robust, transmission mode.
Other features and advantages of the invention will be apparent from the following detailed description and from the claims.